I don't think I've seen Muffet ever say that someone shapeshifted during all of my playthroughs of the game. I do believe it's Mettaton, though. Another thing, it couldn't be Gaster/MysteryMan because only in a specific FUN value does he appear, and if he does, he wouldn't be able to escape the gray, dank room he's forever trapped in. If he somehow DID, then he wouldn't have any money to pay Muffet with, especially within the time frame he's given. Sorry if this reply came off as rude. Btw this account is shared.